1. Field of the Invention
The present invention relates to active noise and vibration cancellation systems, and more particularly, to headsets utilizing active noise cancellation.
2. Description of the Related Art
In various circumstances ambient sound can be disconcerting to, or can create an environment that is uncomfortable or unsafe for, humans. Conventionally, passive headsets or earplugs have been employed in an attempt to reduce the perceived level of ambient noise. In conventional headsets or earplugs, the ambient sound perceived by the wearer is reduced by occlusion of sound from the earpieces and absorption of transmitted sound by materials within the earpieces. The effectiveness of the attenuation depends upon the nature of the ambient noise and the qualities and characteristics of the individual headset or earplugs.
In various applications, however, passive attenuation of sound may be unsatisfactory. Some environments, for example, are simply too noisy for comfort or even safety with only passive earplugs. In other environments, the elimination of extraneous noise is a paramount concern, and satisfactory results cannot be achieved using only passive means. Although the amplitude of the extraneous noise may be significantly diminished, it is almost impossible to completely isolate the wearer from extraneous noise using passive means at low frequencies.
To provide higher quality sound reduction, active noise cancellation headsets attenuate unwanted sound using destructive interference (superposition). Unwanted sound is canceled by propagating anti-noise, identical to the waveform of the unwanted noise but inverted, which interacts with and cancels the unwanted waveform. Anti-noise may be generated by a sound generating actuator driven by a controller. The controller drives the actuator according to signals representative of the noise field to be canceled. More specifically, the residual noise (i.e., the noise remaining after superposition) is sensed, typically by a microphone, and a signal indicative of the residual noise is provided to the controller. The controller drives the actuator accordingly.
Active noise cancellation systems are often susceptible to overload as a result of very low frequency (VLF) disturbances. To generate low frequency anti-noise signals, the actuator (e.g., sound generator) must commonly generate large amplitude signals requiring considerable displacement of the cone or diaphragm of the actuator. Use of sufficiently large actuators, however, is not practical in various small systems. For example, in headsets, mobility and comfort considerations do not permit large displacement actuators. This phenomena is particularly a problem with open-back on-the-ear headsets.. Due to the inherent bass roll-off of such headsets, the pressure level that may be achieved at low frequencies is reduced.
Reduction of the very low frequency output can be attempted by tailoring the loop response of the system to have a steep rate of low frequency roll-off. However, the approach is not practical; steep roll-off loop responses are usually accompanied by instability.
One solution is to move the loop response low frequency cutoff frequency higher and use only a moderate increase in roll-off rate. However, this approach reduces the amount of low frequency cancellation which can be achieved within the audio band, thus reducing the overall effectiveness of the noise cancellation system.